Method and apparatus for generating highly repetitive pulsed plasmas

ABSTRACT

A pulsed radio frequency inductive plasma source and method are provided. The source may generate plasma at gas pressures from 1 torr to 2000 torr. By utilizing high power RF generation from fast solid state switches such as Insulated-Gate Bipolar Transistor (IGBT) combined with the resonance circuit, large inductive voltages can be applied to RF antennas to allow rapid gas breakdown from 1-100 μs. After initial breakdown, the same set of switches or an additional rf pulsed power systems are utilized to deliver large amount of rf power, between 10 kW to 10 MW, to the plasmas during the pulse duration of 10 μs-10 ms. In addition, several methods and apparatus for controlling the pulse power delivery, timing gas and materials supply, constructing reactor and substrate structure, and operating pumping system and plasma activated reactive materials delivery system will be disclosed. When combined with the pulsed plasma generation, these apparatuses and the methods can greatly improve the applicability and the efficacy of the industrial plasma processing.

RELATED CASES/PRIORITY CLAIMS

This application claims the benefit under 35 USC 119(e) to U.S.Provisional Patent Application Ser. No. 61/801,131, filed on Mar. 15,2013 and entitled “Method and Apparatus for Generating Highly RepetitivePulsed Plasmas”, the entirety of which is incorporated herein byreference.

APPENDIX

Appendix A (6 pages) is a listing of the numbered elements in thediagrams. Appendix A forms part of the specification and is incorporatedherein by reference.

FIELD

The disclosure is related to the highly repetitive radio frequencypulsed inductive plasma generation at high gas pressures. In addition,the disclosure is related to the operation of said pulsed plasma systemin its applications to the materials processing such as nano scalematerial manufacturing, toxic chemical processing, plasma assistedmaterial deposition and coating, surface removal, surface activation andsurface property modification. The pulsed plasma system may also be usedfor nanodevice fabrication such as the selected activation, deposition,removal of nanomaterials such as nanowires, nanoparticles, quantum dots,nanophosphors—on different substrates and surfaces.

BACKGROUND

Plasma processing has been used in many industrial applications such asplasma etching, thin film deposition, ion implantation, surfacemodification, and others due to its ability to convert electrical powerinto superior chemical/thermal reactivity. While plasma device operatesin a wide range of gas pressures, the majority of plasma sources havebeen operated in a vacuum or at low gas pressure. A plasma device mayuse radio frequency (RF) power to generate the plasma and there are manydifferent plasma sources such as RF capacitive discharge, RF inductivedischarge, transformer coupled plasmas (TCP), and helicon sources thatcan operate in the low gas pressure condition. While these plasmasources have been successfully utilized in semiconductor chipmanufacturing and vacuum thin film coating, the low gas pressureoperation has limited the use of plasma tools to situations in which ahigh throughput is not critical. In comparison, many industrialprocesses requires the material processing throughput to be on the orderof hundreds of grams per hour or square meters per minute. Furthermore,in the case of nanotechnology applications, a high rate of reactiveradical generation is critical for industrial scale process due to alarge surface area of nanomaterials. Because the low pressure plasmasource starts with a smaller number of molecules, the reactive speciesgeneration is limited and the reaction throughput is difficult to scaleup.

One way to overcome the throughput issue is to utilize a plasma sourcethat operates at high gas pressures between 1 torr and 2,000 torr. Forexample, RF inductive plasma generation at atmospheric pressure has beenaround since 1960s. In addition, DC and AC arc plasmas operate atatmospheric pressure range and are used for thermal plasma spray, arcwelding, arc deposition and others type of applications requiring highthermal reactivity. The technical challenge of these high pressureplasma sources are their inherent tendency to operate at a high gastemperature at 2,000 C or higher when the input power level is increasedto above 10 W/cc level in order to increase the reactive speciesgeneration and to achieve high throughput. In the case of RF highpressure discharge, well known alpha-to-gamma state transitionhighlights this tendency. This is due to the fact of very high collisionrates between the electrons and the gas molecules (described in Alpha toGamma mode transition: “Radio-Frequency Capacitive Discharges”, by YuriP. Raizer and Mikhail N. Shneider, CRC Press, 1995) resulted in eventualgas heating to high temperature. As such, applications of plasma sourcesoperating at high gas pressure have been limited to high temperaturematerials processing. Arc deposition can provide high quality coatingsuch as Titanium nitride on to metal cylinders but not on to theflexible polymer surface.

If the plasma operation is in steady-state, the only ways to limit andcontrol the gas heating by plasma are to use fast gas flow to limit thetime that the gas spends in the plasma volume or to deploy activelycooled plasma facing components to set up a thermal loss boundary asdescribed in U.S. Pat. No. 8,013,269 and “An atmospheric pressure plasmasource”, by Jaeyoung Park, I. Henins, H. Herrmann, and G. Selwyn, J.Jeong, R. Hicks, D. Shim and C. Chang, Applied Physics Letter, Volume76, 288, 2000. For a small-scale system, it may be possible to utilizethese two methods to alleviate the inherent plasma gas heating problem.However, to scale up of these methods to alleviate the plasma gasheating problem become too complex and costly if a large scale system isconsidered. Specifically, fast gas flow system requires expensive gasrecovery system. In addition, it is challenging to maintain uniformtransit time across the entire plasma reactor volume. Furthermore, theheating rate will increase by volume while the loss boundary willincrease only by surface area. Thus, as the system size is increased, acomplex and costly cooling system is required in order to maintain itsstable operation and to reduce the thermal damage to plasma facingcomponents.

In scholarly articles, there has been some amount of work on generatingpulsed plasmas in high gas pressures including the pulsed DC arc, pulsedmicrowave and even pulsed laser plasma generation. For example, see U.S.Pat. No. 3,995,138; “Pulsed microwave plasma polymerization of siliconoxide films: Application of efficient permeation barriers onpolyethylene terephthalate”, by Michael Deilmann, Sebastian Theiβ, PeterAwakowicz, Surface and Coatings Technology, Volume 202, 1911, 2008; and“Laser-induced breakdown by impact ionization in SiO2 with pulse widthsfrom 7 ns to 150 fs”, D Du, X Liu, G Korn, J Squier, and G Mourou,Applied Physics Letters, Volume 64, 3071, 1994. However, highlyrepetitive pulsed RF plasma generation has not been pursued due to thedifficulty of coupling proper RF power to the plasmas for short periodwith high repetition rates.

There are two main reasons for this. The first reason is a physicsissue. The collisions between plasma particles (i.e. ions and electrons)and the neutral gas particles become very frequent at high gas pressure.For example, at 10 torr of argon gas, electron-argon elastic collisionfrequency is about 26 GHz. As a result, electrons are not magnetized ingeneral at such high gas pressure. At 10 torr of argon, electron-argoncollisions are too frequent for electrons to complete even one cyclotronmotion between collisions unless the magnetic field strength exceeds 1Tesla, which is hard to generate in a large volume. Under thenon-magnetized electrons at high gas pressure condition, most of plasmawaves cannot be excited due the collisional damping with neutral gas.The only allowable plasma waves are Langmuir wave (or plasmaoscillation) for electrons, ion acoustic wave (or sound wave) for ionsand electromagnetic light wave as described in “Waves in Plasmas” byThomas Stix, Springer, 1992. Since none of those waves are easily usablefor power coupling from externally applied RF fields to the plasmas, thehigh gas pressure conditions make the RF inductive plasma generationtechnically challenging. In comparison, DC/AC arcs utilize physicalelectrodes to generate the plasma, thus a lack of available plasma wavesis not an issue.

The second reason is engineering and technology in nature. Specifically,there is a lack of readily available high power RF power supplies and RFtuning systems to deliver very large power to antenna for very shortperiod of time. Typical RF power generator operates at 13.56 MHz anddelivers the steady-state power output of 1-10 kW into 50 ohm load. Thismeans that a typical RF power system operates with the maximum currentrating of 10-20 A and the maximum voltage 500-1000V. Since the resonancecircuits, whether in parallel or in series, can only increase eithercurrent or voltage but not both at the same time, it is technicallychallenging to initiate or to sustain high power coupling to plasmaabove 10 kW, especially for a short pulse duration of less than 10 mswhich complicates adjustment in the tuning circuit. In addition, as willbe discussed in the experimental section, the time lag to initiate thegas breakdown from the RF power onset increases from less than 1microsecond to 100 microseconds or more with increasing gas pressure fora fixed antenna voltage and current. In the case of a plasma sourceoperating in argon gas, the necessary EMF voltage is approximately 500 Vper centimeter of reactor circumference at 10 torr for a reactor size of5 centimeter diameter. Since these level of EMF voltage cannot be easilygenerated by readily available RF power generator and the availabletuning circuit, a different approach is necessary to initiate, maintainand control the highly repetitive short pulse RF plasmas at high gaspressures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an embodiment of a radio frequency pulsed inductiveplasma reactor;

FIGS. 1B1 and 1B2 are a top view and a side view, respectively, of theradio frequency pulsed inductive plasma reactor in FIG. 1;

FIG. 2 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor;

FIG. 3A illustrates an embodiment of the radio frequency pulsedinductive plasma reactor with a substrate;

FIG. 3B illustrates an embodiment of the radio frequency pulsedinductive plasma reactor with a substrate and a lock load system;

FIG. 4 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor having a material collection system;

FIG. 5 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor;

FIG. 6A illustrates another embodiment of the radio frequency pulsedinductive plasma reactor that has dual pulsed RF power systems;

FIGS. 6B1 and 6B2 are a top view and side view, respectively. of theplasma reactor in FIG. 6A;

FIG. 6C illustrates another embodiment of the radio frequency pulsedinductive plasma reactor that has antennas and a single pulsed RF powersystem;

FIGS. 7A-7E illustrate another embodiment of the radio frequency pulsedinductive plasma reactor, termed as “pulsed plasma spray device”, thattransports the activated materials from the plasma reactor to the targetsurfaces located outside the reactor;

FIG. 8 shows Paschen curve data for the radio frequency pulsed inductiveplasma reactor;

FIG. 9 shows particle in cell simulation results with breakdown delaytime as a function of azimuthal electrical strength of the antenna forgas pressure in argon using 1 MHz RF power;

FIG. 10A-10D show experimental results for the pulse plasma reactor;

FIGS. 11A-11C show examples of a resonance circuit that may be used inthe pulsed plasma reactor; and

FIG. 12 illustrates an hourglass shaped reactor chamber embodiment ofthe pulsed plasma reactor.

DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS

The disclosure is particularly applicable to pulsed RF inductive plasmasystem and method and it is in this context that the disclosure will bedescribed. It will be appreciated, however, that the system and methodhas greater utility since it may be used for materials processing suchas nano scale material and device manufacturing, toxic chemicalprocessing, plasma assisted material deposition and coating, surfaceremoval, surface activation and surface property modification. Inaddition, the pulsed plasma system may also be used for nanodevicefabrication such as the selected activation, deposition, removal ofnanomaterials, such as nanowires, nanoparticles, quantum dots,nanophosphors, on different substrates and surfaces. The nanodevicefabrication may be done on surfaces including Silicon, SiC, AN, GaN,Sapphire as well as glass, plastics, polymers, fabric, paper,fiberglass, composite materials, metals and alloys. The nanodevices maybe formed on both flexible and rigid substrate and surfaces.Furthermore, porous and absorbent materials may be used as well as solidmaterials.

A pulsed RF plasma source described below may limit the duration ofenergy transfer between the plasma electrons at 1-10 eV range to the gasmolecules by directly controlling the plasma source duration. Forexample, the range of pulse duration may be 10 μs-10 ms. Though thecollision frequency between the electrons and the gas molecules isfrequent, as much as 26 GHz at 10 torr of Argon gases, the efficiency ofenergy transfer rate is reduced due to a large mass difference betweenthe electron and the argon atom. As such, the gas molecule heatingcaused by electron can be limited and controlled by such pulse duration.In addition, the heat transfer from the plasmas including the electronsand the heated molecules to the surrounding structure is gradual andrequires sufficient time to build up the thermal effects. By keeping thepulse duration less than 10 ms, the thermal effects to the plasma facingcomponents are greatly reduced. By repetitively pulsing the RF plasmaoperation at 1 Hz to 1,000 Hz, it is then possible to limit the thermalproblems without sacrificing the desired reaction throughput.

The pulsed radio frequency (RF) inductive plasma source operating athigh gas pressures has advantages over conventional steady-state RFplasma sources with improved efficiency of generating plasma reactivityand with reduced thermal damages to plasma facing components. Since ahigh reaction throughput is critical for industrial applications ofplasma source for materials processing including nanotechnologyapplications, it is beneficial to generate pulsed RF plasma at gaspressures from 1 torr to 2,000 torr and preferable from 5 torr to 2,000torr, resulting in high plasma densities between 10¹⁵ cm⁻³ and 10¹⁷ cm⁻³during the pulse. At such plasma densities, plasmas can generate copiousamounts of reactive radicals from a wide range of precursor materials byrapid thermal, chemical and electrical energy transfer from plasmaelectrons to the precursor materials. In comparison, many of the RFinductive plasmas used at semiconductor materials processing operatesfrom 1 mtorr to 50 mtorr pressure range with the plasma density between10¹⁰ cm⁻³ and 10¹² cm⁻³, resulting in much lower reactivity than oneavailable at high pressure plasma sources. At such high gas pressures,however, frequent collisions between plasma electrons and neutral gasmolecules greatly limit the available modes of plasma wave propagation.Under this condition, it has been difficult to efficiently couple RFpower into plasma in a short pulse mode, where the pulse duration isbetween 10 μs-10 ms. This particular range of pulse duration isimportant to materialize the key benefits of pulsed plasma operation. Byutilizing high power RF generation from fast solid state switches suchas Insulated-Gate Bipolar Transistor (IGBT) combined with the resonancecircuit, large inductive voltages are applied to RF antennas to allowrapid gas breakdown from 0.1-100 μs. After initial breakdown, the sameset of switches or additional RF pulsed power system is utilized todeliver large amount of RF power, between 10 kW to 10 MW, to the plasmasduring the pulse duration of 10 μs-10 ms. In addition, several methodsand apparatus of controlling the pulse power delivery, timing gas andmaterials supply, constructing reactor and substrate structure, andoperating pumping system and plasma activated reactive materialsdelivery system will be disclosed. When combined with the pulsed plasmageneration, these apparatuses and the methods can greatly improve theapplicability and the efficacy of the industrial plasma processing.

FIG. 1A illustrates a first embodiment of a radio frequency pulsedinductive plasma reactor that generates an radio frequency inductiveplasma at gas pressures from 1 torr to 2000 torr (and preferably from 5torr to 2000 torr) and FIGS. 1B1 and 1B2 are a top view and a side view,respectively, of the radio frequency pulsed inductive plasma reactor inFIG. 1A. The reactor may have a reactor chamber 102 that may have aparticular shape, such as a cylinder in FIG. 1B or an hourglass shape asshown in FIG. 12, and may also have an inlet region at one end and anoutlet at an opposite end of the chamber. The reactor chamber may be ofany shape and may be for example, square, rectangular, oval or ahexagon. In each of the embodiments, the reactor chamber may have adiameter of between 0.5 inches to 12 inches. The thickness of the wallsof the reactor chamber may be 0.5 mm to 5 mm depending on the propertiesof the material used for the walls. The reactor chamber walls may alsobe made of two or more materials as long as a dielectric (non-metal)material is used adjacent the antenna. The reactor may also have apulsed radio frequency generator that is coupled to the reactor chamber102. The pulsed radio frequency generator may further include an antenna107 and a pulsed radio frequency source 108 (that generates a pulsedsignal 109) that is coupled to the antenna. The antenna 107 may surrounda portion of the reactor chamber 102. In one implementation, the antenna107 may be a multi-turn coil that encircles the portion of the reactorchamber 102 as shown in FIGS. 1A, 1B1 and 1B2.

The reactor may also have an inlet 101 that has an entry point for acarrier gas and one or more reactive precursors materials that areturned into a plasma in the reactor chamber. The reactor may also have apressurizing system 104 that may be used to maintain the gas pressure inthe reactor chamber. The pressuring system may further include a pump106 and a valve 105. In this embodiment, any precursor material (a gas,a liquid or solid precursors) may be used and the pumping may becontinuous or operated using a timed pulse. For example, the precursormaterial may be a solid precursor material having a linear size ofbetween 10 nm to 0.1 mm and preferably a linear size larger than 10 nmand less than 0.1 mm. As another example, the precursor material may bea liquid precursor material having a linear size of between 10 nm to 0.1mm and preferably a linear size larger than 10 nm and less than 0.1 mm.In one example, the one or more reactive precursor materials may be, forexample, a reactive gas containing hydrogen, oxygen, nitrogen, fluorine,chlorine, sulfur, phosphor and hydrocarbon. In another example, the oneor more reactive precursor materials may be acids, bases, polymers,metals, ceramics, and composite materials.

In operation, the carrier gas and the one or more reactive precursorsmay be introduced into the reactor chamber at the inlet 101 and thepressurizing system may maintain a pressure in the reactor chamber of 1torr to 2000 torr. Then, a pulsed radio frequency signal is generated bythe pulsed radio frequency source 108 and that signal is coupled to theantenna 107 which initiates a breakdown of the carrier gas and one ormore reactive precursors and generates a plasma due to the breakdown ofthe carrier gas and one or more reactive precursors.

Thus, the reactor generates a pulsed plasma. The pulse plasma means thatthe duration of plasma generation is short compared to other relevanttime scales for plasma source operation. Two specific time scales arechosen to define the pulse plasma operation. One is a time scalerelevant to the thermal damage to the RF source structures withoutcomplex active cooling systems. Typically, the plasma generationinvolves significant heat generation in the plasma medium and subsequentheat transfer to surrounding structures, including but not limited tothe antenna, an enclosure of the pulsed radio frequency generator, oneor more walls of the reactor chamber, a substrate for surface treatment,nozzles in the inlet, a material collection system such as filter andcollectors. By limiting the pulse duration of the plasma sourceoperation, it is then possible to alleviate or eliminate coolingrequirements to the affected surfaces, thus simplifying the engineeringrequirement and improving the reliability and overall power efficiency.In addition, the plasma reactor can operate to generate reactive plumeonto various target surfaces for deposition, coating, surface removal,surface modification and treatment. By operating in pulsed manner, theamount of thermal energy per each pulse can be limited, so eventhermally sensitive targets can be utilized. Examples will be plastics,polymers, fiberglass, fabric, ceramics, glass, and even papers as wellas metals, alloys and composite materials. Specifically, the reactor 100may implement a plasma pulse operation from 10 μs to 10 ms, whereprevious intermittent plasma source operation using RF power cannotadequately address due to the RF power supply and plasma couplinglimitation.

The second time scale is the time scale relevant to a transit time ofthe gas flow across/through the plasma reactor. In most of the plasmaprocessing applications, there is gas flow that carries precursormaterials in various phases such as gas, liquid droplets, and solidparticles into the plasma reaction volume. During their transit timepassing through the plasma reaction volume (for example, in the portionof the reactor chamber that is surrounded by the antenna), the precursormaterials receive thermal, chemical, electrical energies from theplasmas and undergo desirable reactions. In the case of steady-stateplasmas operating at a fixed power level, the control of the precursormaterials reactivity is governed by the gas flow speed, which isdifficult to control precisely over the entire reaction volume asdescribed in “Nanoparticle formation using a plasma expansion process”,by N. Rao, S. Girshick, J. Heberlein, P. McMurry, S. Jones, D. Hansen,and B. Micheel, Plasma Chemistry and Plasma Processing, Volume 15, 581,1995 that is incorporated herein by reference. Additionally, in the caseof high power, high throughput plasma reactors, the gas flow needs to bevery large, on the order of 100 liters per minute or more, in order tocontrol the plasma reactivity that is described in U.S. Pat. No.6,994,837 that is incorporated herein by reference. In comparison, thepulsed plasma operation allows reliable and accurate control of theprecursor material reactivity by controlling the pulse duration, whenthe pulse duration is comparable or shorter than the gas transit time.In the case of gas flow moving at a speed of 1 m/s to 10 m/s, thetypical transit time is about 1 ms to 10 ms for a 10 cm length plasmareactor. Thus, the pulse plasma operation from 10 μs to 10 ms providessimple and reliable control of the precursor material reactivity byregulating the thermal, chemical and electrical energy transfer from theplasmas.

Energy and Power Input During Pulses

At high gas pressure, even a modest reactor volume contains a largenumber of gas atoms and molecules that will get ionized and turned intoa plasma. In the case of argon gas pressures at 5 torr and at 50 torr,the corresponding gas number densities are 1.4×10¹⁷ cm⁻³ and 1.4×10¹⁸cm⁻³. For a plasma reactor volume of 100 cm³, the amounts of gas atomsin the reactor are 1.4×10¹⁹ and 1.4×10²⁰ respectively (or 2.3×10⁻⁵ moleand 2.3×10⁻⁴ mole). In order to generate plasmas, electrons need toprovide sufficient energies to ionize the neutral gas. In the case ofargon, this ionization energy is 15.8 eV for the first electron removal.In general, however, a plasma source requires typically 100 eV-500 eV ofenergy cost for ionization (1 eV=1.6×10⁻¹⁹ J) of one pair of an electronand an ion from a gas atom as described in “Modeling of Air PlasmaGeneration by Repetitive High-Voltage Nanosecond Pulses”, by S.Macheret, M. Shneider, and R. Miles, IEEE transactions on plasmasciences, Volume 30, 1301, 2002 that is incorporated herein byreference. This is because there are many energy loss channels forelectrons in the plasma system, in addition to ionization. The losschannels may include electron impact excitation followed radiation loss,electron loss to the surrounding boundary, and electron heating ofneutral gas atoms by collisions. Assuming an energy budget of 150 eV, inorder to ionize a 5% of gas in the reactor volume, an energy input frompulsed RF power system to the plasmas needs to be 17 J for the reactorvolume of 100 cm³ and a gas pressure of 5 torr, which results in theplasma density of 7×10¹⁵ cm⁻³. In the case of 50 torr gas operation, theamount of energy input needs to be 170 J for the reactor volume of 100cm³, which results in the plasma density of 7×10¹⁶ cm⁻³. For the pulseduration of 10 μs, the required pulse powers are 1.7 MW for 5 torr and17 MW for 50 torr. For longer pulse duration of 1 ms, the required pulsepowers are 17 kW for 5 torr and 170 kW for 50 torr. It is noted thatadditional input powers may be needed in order to generate thermal andchemical reactivity to the precursor materials, by raising temperaturesof precursor materials, by dissociating and decomposing precursormaterials and generating reactive radicals from precursor materials. Assuch, high RF power coupling to the plasma during the pulse is neededfor the RF pulsed plasmas to be useful in plasma materials processing.In this disclosure, we are concerned with the RF powers between 10 kWand 10 MW during the pulse.

Precursor Materials

High power RF pulse plasmas can utilize a wide range of precursormaterials. As shown above, the pulsed RF plasmas can generate very highplasma densities in the range of 10¹⁵ cm⁻³ and 10¹⁷ cm⁻³ during thepulse, if properly powered and controlled. These high plasma densitiesare what make the pulsed RF plasma reactor very attractive for a widerange of plasma applications. In comparison, typical plasma densities ofvacuum based plasma sources such as RF ICP, RF TCP, and RF capacitivedischarges are between 1×10¹⁰ cm⁻³ and 1×10¹² cm⁻³. Separately,spatially averaged plasma densities are typically less than 1×10¹⁰ cm⁻³in corona discharges and dielectric barrier discharges operating atatmospheric pressure. Atmospheric pressure glow discharges using RF orAC power typically operate up to 1×10¹² cm⁻³ before collapsing into alocalized gamma mode, where the plasma generation is limited into thecurrent channel. At the operating densities in the range of 10¹⁵ cm⁻³and 10¹⁷ cm⁻³, the pulsed rf plasmas in this invention have comparableplasma densities to various thermal plasmas such as DC arcs, AC arcs,and thermal induction plasma torches operating with rf power. It isnoted that the operating mode of DC arcs, AC arcs and thermal inductionplasma torches are either steady state or long pulse operation with thepulse duration much greater than 10 ms. For example, the thermalinduction plasma torch is a mature technology producing the steady-stateplasmas with densities in the range of 10¹⁴ cm⁻³ and 10¹⁷ cm⁻³, alongwith the gas temperatures between 3,000 C and 7,000 C, generating highthermal and chemical reactivity as described in Tekna Plasma SystemsInc., Sherbrooke, Quebec, Canada (www.tekna.com) which is incorporatedherein by reference. The induction plasma torch has found nicheindustrial applications in nano-powder and nanomaterial synthesis usinggas and solid precursor materials. However, the application of theinduction plasma torch along with DC and AC arcs, has been severelylimited due to its high temperature operation in steady state, whichresults in very large thermal load to the surrounding structures andlimits its application to the plasma processing with the materialscompatible with high temperature. In comparison, the pulsed RF plasmareactor can minimize inefficiencies and challenges related to high rateof heat generation and dissipation of the steady-state thermaldischarges by utilizing short pulse plasma generation. On the otherhand, as will be discussed shortly in the section, “Plasma Gas Heating”,the pulsed RF plasma reactor operating with plasma densities, in therange of 10¹⁵ cm⁻³ and 10¹⁷ cm⁻³ and with input powers between 10 kW and10 MW, can generate high gas temperature and associated high chemicalreactivity comparable to those steady-state thermal plasmas during thepulse duration in a controlled manner. As such, the RF pulsed plasmascan utilize a wide range of precursor materials from reactive gasescontaining oxygen, nitrogen, fluorine, chlorine, sulfur, phosphor tohydrocarbon, acids, base, polymers, metals, ceramics, and compositematerials in any phases of gas, liquid droplets and solid particles.Since the solid particle precursors up to 100 μm have been used in theinduction thermal plasmas operating between 50 kW and 1 MW, it isprojected that the similar particle size up to 100 μm can also beutilized in the pulsed rf plasma reactor

The use of liquid droplets and solid particles as precursor materialsare particularly useful for industrial applications for metal and hightemperature ceramic and composite materials since it reduces oreliminates the use of highly toxic and reactive organometallic gasprecursors. The use of liquid droplets and solid particles also reducesthe processing complexities related to maintaining the proper chemicalcomposition or stochiometry of the materials. For example, one of thechallenges of the thin film solar cells using copper indium galliumselenide (CIGS) is the proper ratio among copper, indium, gallium andselenide during the deposition process. The pulsed plasma reactor allowsthe use of chemically complex precursor materials in its solid or liquidfrom for surface deposition and coating. The same is true for synthesisand applications of YAG (Yttrium aluminium game) phosphor. In addition,the pulsed operation of the pulsed RF plasma reactor provides a means tocontrol the amount of activated precursors materials by controlling theenergy transfer between the plasma electrons and the precursor materialsusing the pulse duration.

Separately, the high plasma density and subsequent high rate of reactivespecies generation is important in the area of nanotechnology includingnanomaterial manufacturing and nano-device fabrication using plasmas.The key issue is large surface areas of the nano materials in thenanotechnology applications. Without high rates of reactive speciesgeneration, the nano-device fabrication utilizing selected activation,deposition, removal, and patterning of nanomaterials and the structurescannot proceed at rate required for industrial scale. As such, lowpressure plasmas and catalytic reaction path are not well suited forindustrial scale nano-device fabrication, which is one of the reasonsthat nanotechnology adoption is still nascent in the industrialapplications.

Pulsed Operation

Repetitive pulse operation is critical to ensure high thermal andchemical throughput of the plasma reactor. In general, a duty factor ofthe pulsed operation should be in the range of 1-10% level to maintainreasonable degree of throughput. For the pulse duration of 10 μs to 10ms, 1% duty factor corresponds to repetition rate of 1 Hz-1 kHz. Inorder to provide such repetition rate, IGBT is the most suitable solidstate switch at present time though other types of solid state switchessuch as IGCT or GTO may be used.

RF Inductive Plasma Generation at High Gas Pressure

RF inductive plasma system has advantages over other plasma generationsystem. One of them is the non-contact nature of plasma power coupling.In the RF inductive system, the plasma is generated by the RF power fromthe external antenna outside the plasma reactor wall. In comparison, DCand AC plasma generation requires power electrodes to be in contact withthe plasmas. Without the exposed power electrodes, there is no plasmadamage to the electrodes, thus providing better reliability and reducingmaterials contamination. The RF frequency for plasma power coupling inthe disclosure is given between 50 kHz to 10 MHz. At a short pulseduration between 10 μs-200 μs, high RF frequency is important forreactivity control by pulse duration. At 1 MHz RF frequency, the controlof pulse duration by a number of RF cycle is 1 μs increment. At 10 μstotal pulse duration, this means that the pulsed plasma can control itsreactivity within 10%, assuming linear increase in reactivity with pulseduration. If the frequency of RF power is decreased to 100 kHz, it isnot possible to provide proper control of reactivity for the 10 μspulse. It is thus desirable to operate the RF power period between 0.5%and 10% of total pulse duration. For longer pulse duration above 200 μs,the more relevant consideration is the efficiency of RF inductive powercoupling between the antenna and the plasmas. For a fixed antennainductance L, the maximum available instantaneous power on the antennais given as I*L*dI/dt or 6.28*L*I²*f, where I is the current flow in theantenna and f is the frequency of the RF power. Assuming an antennainductance of 1 μH, in order to provide 500 kW of peak power, therequired RF current is 280 A at 1 MHz RF frequency. If the RF frequencyis decreased to 100 kHz for the same antenna, the required current toprovide the same 500 kW is now 900 A. Higher currents in RF power systemusually resulted in higher energy loss due to resistive powerdissipation in cables, antenna and switches as well significantelectromagnetic interference related to parasite inductances in thesystem. As such, it is desirable to keep the RF frequency at least above50 kHz range in the case of high power RF pulsed switching system. Thisfrequency range between 50 kHz to 10 MHz is well suited for the solidstate switching power system using IGBTs, which can deliver very largeRF pulse power from 10 kW to 10 MW for a short pulse duration of 10μs-10 ms.

Plasma Breakdown

One of the technical challenges for pulsed RF plasma generation at highgas pressure is the difficulty of gas breakdown by RF power. In 1889,Paschen published a paper about the breakdown voltage with respect togas pressure and the electrode spacing, which later became the basis of“Paschen's law” or “Paschen curve”. As seen in FIG. 8, the Paschen curvefor argon indicates the required breakdown voltage increases with thegas pressure for a given electrode spacing. Since the RF inductiveplasma does not have physical electrodes, it is necessary to convert therelevant parameters. In the case of cylindrical RF antenna, the relevanty-axis scale would be the antenna voltage or azimuthal electric fieldsmultiplied by the antenna circumference. Since the breakdown voltage inFIG. 8 is given by the electric field value multiplied with theelectrode spacing between the parallel pate, it is necessary to multiplya factor of 2π or 6.28 to the published parallel plate breakdownvoltage, in order to estimate the proper breakdown voltage needed forthe for the cylindrical RF antenna configuration. As for the x-axis, oneneed to use the radius of the cylindrical antenna to estimate the “pd”value in the Paschen curve. So, in the case of a cylindrical antennawith a radius of 2.54 cm (or 1 inch), the pd value then becomes 12.7torr cm at 5 torr argon pressure and the pd value becomes 127 torr cm at50 torr argon pressure. For a parallel plate configuration, the requiredbreakdown voltages are 600 Volt for 12.7 torr cm and 4,000 Volt for 127torr cm. After the required conversion factor of 2π or 6.28 to properlyaccount for the cylindrical geometry, it is clear that one needs veryhigh antenna voltages to achieve breakdown, 3,600 V for 12.7 torr cm and25 kV for 127 torr cm. Moreover, it is noted that the Paschen curverepresent a minimum breakdown voltage, not the required voltage tobreakdown the gas rapidly within 1-100 μs time scale. Typically, oneneeds a factor of 1.5 to 3 higher voltages to the antenna above thePaschen breakdown voltage within 1-100 μs time scale in order to ensurerapid and reliable breakdown.

Electrical Circuit to Provide High Voltages to the Antenna Using RFPulse Power

Gas breakdown at high pressures requires high voltage across the RFantenna. Based on the simple estimate from Paschen curve, the antennavoltage needs to be between 3.5 kV and 25 kV for the gas breakdownbetween 5 torr and 50 torr gas pressure for the reactor size of 2 inchdiameter. The pulsed plasma reactor may use an RF electrical circuitusing solid state switches and a cylindrical coil antenna configurationthat can generate an antenna voltage of 20 kV at a RF frequency of 1MHz. For a given antenna, the voltage across the antenna is given asV_(antenna)=6.28f*L_(antenna)*I_(antenna), where V_(antenna) is thevoltage across the antenna, f is the RF frequency, L_(antenna) is theantenna inductance, and I_(antenna) is the antenna current. Since theantenna voltage increases with the antenna inductance for a fixedcurrent, it is advantageous to utilize an antenna with sufficientinductance to provide the breakdown voltage. For example, an inductanceis about 1.0 μH for a 6 turn cylindrical coil made of 6 mm copper tubewith 50 mm diameter and with 10 mm pitch, corresponding to the coillength of 60 mm. This coil will encompass about 120 cm³ of reactorvolume. In order to generate 20 kV across this antenna at 1 MHz of RFfrequency, the antenna current needs to be 3.2 kA. For example, a seriesresonance circuit can be used, as shown in FIG. 11A, with the resonancecapacitor to minimize the impedance in order to flow this much current.At 1 MHz resonance, resonance capacitor value is 25 nF, sincef_(resonance)=1/(6.28*L^(0.5) C^(0.5)). Typically, the series resonancecircuit has a finite resistance on the order of 0.1-0.2 ohm from thevarious electrical components such as wires, connectors and switches,prior to plasma loading. A quality factor, Q, of this circuit becomes 63for a series resistor of 0.1 ohm, since Q=6.28*f_(resonance)*L/R⁰_(series), where f_(resonance) is 1 MHz, L is 1 μH, and R⁰ _(series) is0.1 ohm. Therefore, this series resonance can be driven by the RFswitching system with a RF voltage of 320 V and a RF current of 3.2 kA.This RF system can be readily built by existing commercial IGBT. Notethat in this case, a circulating RF power is ˜1 MW, assuming the voltageand the current values are in rms. For example, IXYS corporation offersa discrete IGBT with a voltage rating of 1200V rating, a current ratingof 50-100 A and fast switching time of 43 ns, see part numberIXYH50N120C3. By utilizing multiple IGBTs such as the above IGBTs fromIXYS, it is then possible to construct RF pulse power system that candeliver 3.2 kA at 320V. Since the required RF pulse duration is shortbetween 10 μs-10 ms, and thermal failure of IGBTs can be easily avoidedby employing sufficient number of IGBTs, 32 or 64, with proper cooling.It is also possible to utilize a parallel resonance circuit (shown inFIG. 11B) to drive this antenna for the necessary voltage. However, inthe case of parallel resonance circuit, one must provide the entire 20kV from the RF switching system, while the current requirement will bemuch reduced. The optimal configuration would then be to utilize theseries connection of IGBTs to provide sufficient voltage to the RFantenna, which is also a possible RF switching power systemconfiguration. Though the choice of RF resonance circuits depends onmany things, it is noted that a series resonance circuit is bettersuited because of the inherent robustness of parallel connection ofIGBTs, compared to series connection of IGBTs. The overvoltage failuremode of solid state switch is much more challenging than overcurrentfailure mode. Especially for short pulse operation described in thecurrent invention, the IGBT system can handle larger than rated current,while the overvoltage failure is independent of the pulse duration. Itis also noted that hybrid circuit combining the benefits of the seriesresonance circuit and the parallel resonance circuit can be used incombination as shown in FIG. 11C.

Plasma Sustainment and RF Power Delivery at High Gas Pressures

After breakdown, the interaction between the plasmas and the RF antennachanges dramatically. This is because the gas medium is dielectric innature with the appropriate dielectric constant of 1, while the plasmamedium is electrical conductor with its conductivity comparable tocopper. Since there is no appropriate plasma wave mode at high gaspressures, the RF power coupling to the plasma can be viewed asinduction heating after breakdown. From the RF circuit point of view,this means there will be real resistance component in the RF circuitfrom the power coupling to the plasmas and decreased inductance ofantenna from the induced current in the plasmas. There are three ways tohandle this change in RF circuit composition. The first way is to donothing and then the series or parallel resonance circuit will be out ofresonance due to the reduced inductance of the antenna. If the change inthe inductance is small compared its vacuum value, it is then stillpossible to maintain a large current in the series resonance circuit andto deliver high RF power to the plasmas. A second way is to change theswitching frequency of the IGBTs to compensate the plasma coupling asshown in FIG. 5 and it is very useful to utilize the solid stateswitching RF powers since the RF frequency can be dynamically adjustedduring the pulse by providing proper gate timing. In comparison, thismethod of change RF frequency during the pulse operation is verydifficult in the case of vacuum tube based RF power system. In fact, insteady-state RF plasmas, typically a tuning circuit changes thecapacitor value once the gas breakdown is achieved to compensate thechange in antenna inductance. This second way, however, has somepotential downside risk that needs to be considered. It is because thefrequency shift of RF switching system is always upward since theresonance frequency is given as f_(resonance)=1/(6.28*L^(0.5) C^(0.5)),while the inductance is decreased. At higher frequency, RF power cannotpenetrate the deep into the plasma reactor as well due to a phenomenacalled, “skin effect”. If this skin effect is severe and RF coupling tothe plasma is very localized, one can use the third approach. In thethird approach, a separate set of RF antenna is deployed in the plasmareactor, preferably at the same axial location of the 1^(st) antenna,and this 2^(nd) antenna handles the plasma sustainment and the RF powerdelivery after the breakdown by the 1^(st) antenna. The location of thesecondary antenna has to be overlapped (see FIG. 6A for example) or inclose proximity to the primary power antenna, as shown in FIG. 6A. Thisis because the relatively fast cooling and extinction rate of plasma athigh gas pressure. As for the RF circuit for the 2^(nd) antenna, it ismore advantageous to utilize the parallel resonance circuit, as shown inFIG. 11B. This is because the 2^(nd) antenna does not need high voltageoperation since its operation depends on the existence of plasmas in thereactor by the 1^(st) antenna. Here we provide description of a RFelectrical circuit using solid state switches and cylindrical coilantenna configuration that can generate deliver 1 MW of RF power to 0.5ohm plasma load. An inductance is about 0.2 μH for a 2 turn cylindricalcoil made of 6 mm copper tube with 50 mm diameter and with 15 mm pitch,corresponding to the coil length of 30 mm. This coil will encompassabout 60 cm³ of reactor volume. In this case, we will start with theresonance frequency of 0.4 MHz in order to improve the RF penetrationinto the reactor volume. At 0.4 MHz, the inductive impedance of theantenna is 0.5 ohm. Assuming the real part of antenna resistance is 0.5ohm from the plasma loading, the total impedance of the antenna is 0.5ohm. In order to couple 1 MW of RF power, the RF voltage needs to be 700V and the RF current needs to be 1.4 kA. As previously mentioned, it isrelatively straightforward to construct a RF switching power systemusing IGBTs for those parameters. In fact, this provides two possibleconfiguration to power this 2^(nd) antenna. One is the use of 2^(nd) RFpulsed switching system to operate this 2^(nd) antenna in order todeliver 1 MW of RF power to the plasma load of 0.5 ohm, as shown in FIG.6A. The other is to utilize a single RF pulse power system with twoantenna in a hybrid resonance circuit, as in FIG. 6C and FIG. 11C.Initially the RF pulse power system operates at high frequency, forexample at 1 MHz, and utilizes the series resonance circuit for the1^(st) antenna with high inductance to initiate the plasmas. Afterachieving breakdown, the RF pulse power system can then operate at lowerfrequency, for example at 0.4 MHz, and utilize the parallel resonancecircuit for the 2^(nd) antenna with low inductance to deliver high powerto sustain the plasma. Since the resonance conditions are different forthe series resonance circuit and the parallel resonance circuit, onlyone part of the hybrid circuit is active at each frequency. Eitherconfiguration will be acceptable, though the choice of RF resonancecircuits depends on many things such as cost, system complexities,controllability, efficiency and others.

Plasma Gas Heating

During the pulse, the plasma electrons can transfer its energy to gasmolecules via collisions, resulting in gas heating. While the collisionfrequency is high, the rate of energy transfer efficiency is low foreach collision due to very large discrepancy in mass. The generalexpression for gas heating by plasma electrons is given asdT_(gas)/dt˜n_(e)<σν>T_(e)(m_(e)/m_(gas)), where T_(gas) the gastemperature, n_(e) is the electron density in the plasma, σ is theelectron gas collision cross section, ν is the electron velocity, T_(e)is the electron temperature and m_(e)/m_(gas) is the mass ratio betweenelectron and gas atoms. In the case of argon operating at 10 torr, theplasma electron density is 1.4×10¹⁶ cm⁻³, assuming a 5% ionizationfraction. The plasma density can be controlled by the RF pulse inputpower, while the average electron temperature is between 5 and 10 eV ina wide range of input powers, based on the experimental and theoreticaldatabase of rf thermal plasmas in steady state. It is noted that theplasmas establish its density and temperature equilibrium for a given rfinput power in a very short time, typically within a few μs due to therapid response of electrons to the applied rf electric fields. We willuse 5 eV electron in this example, resulting in electron thermalvelocity of 9.4×10⁷ cm/s and electron temperature of 57,000 Kelvin. At 5eV, electron-gas collision cross section is approximately 1×10⁻¹⁵ cm²for argon gas. By putting the above numbers in the gas heatingexpression, dT_(gas)/dt is 1×10⁸ Kelvin/s or 100 Kelvin/μs. This resultshows that the gas temperature in the pulsed plasma reactor willgradually increase over the pulse duration by electron gas heating. Fora fixed plasma density of 1.4×10¹⁶ cm⁻³, the gas temperature will riseto 500 C (from 20 C inlet temperature) for 5 μs pulse duration and to1,000 C for 10 μs pulse duration, in this simplified estimate. Thus, thepulse duration control provides a powerful yet convenient method toadjust the reactor gas temperature, especially for pulse durationbetween 10 μs and 10 ms. Since the gas temperature is one of thecritical parameters to determine the thermal and chemical reaction ofthe precursors, this results provide the basis for the reactivitycontrol of the pulse plasmas. It is noted that we have ignored theenergy loss mechanism by gas via conduction, convection and radiation,so the temperature rise will be less than this value and the gastemperature will likely saturate between 5,000 C and 10,000 C. It isalso noted that the gas heating rate can be controlled by plasma densityor RF pulse power. Finally, we note that in the case of liquid and solidprecursor materials, the heating rate of those non-gaseous medium willbe less than the one of the gas precursor materials since the onlysurface layer of the liquid and solid materials can absorb the energydirectly from the plasmas.

Pulsed Plasma Operation Heat Generation

The pulse plasma reactor only generates a small amount heat during thepulse operation. It is noted that the heat capacity of the gas is verysmall, 12.6 J/(mole×Kelvin). Since the pulse plasmas heat only a smallamount of gas molecules, 2.3×10⁻⁵ mole of argon gas for 100 cm³ reactorvolume at 5 torr, the total amount of heat from the high temperature isgas is relatively small. Even at 5,000 C, the argon gas in the pulsedreactor will only contain 1.5 J of energy at the above condition. Evenat very high repetition rate of 1 kHz, the total gas heating results in1.5 kW, equivalent to a household hair dryer. This is because of theshort pulse nature of pulsed RF plasma operation. By limiting heatgeneration during the pulse, it is then possible to utilize manythermally sensitive materials as substrate, filter and reactor wallmaterials in the RF pulsed plasmas. Example will be plastics, polymers,fiberglass, fabric, ceramics, glass, and even papers. If a flexiblematerials can be utilized, roll-to-roll plasma surface treatment cangreatly increase the materials throughput compared to batch system.

A repetitively pulsed RF plasma source present a major paradigm changein how the plasma reactor operates with respect to carrier gas andprecursor materials injection, gas pumping and reactor operatingpressures. A similar example is the internal combustion engine with thetimed ignition, fuel injection, compression and exhaust. Since theplasma activation of precursor materials occurs only during the RF powerpulse or during the afterglow phase immediately after the RF powerpulse, it can be beneficial to operate the plasma reactor with timedcarrier gas and precursor materials injection. Separately, the gaspumping may be operated in a pulsed mode in synchronization of pulsed RFpower or pulsed gas and precursor injection. Even the reactor pressurecan be controlled and modulated in time domain with respect to otherpulses in the systems such as RF power, injection and pumping. Thedisclosure introduces the concept of timed pulsed operation among RFpulse power, gas and precursor materials injection, pumping and dynamicreactor pressure control.

The benefits from the pulsed operation of RF power, injection, andpumping reactor pressure are: reduction of materials and electricityusage, increase in reaction throughputs. In addition, the timing controlamong different pulses represents a powerful control tool for plasmareactivity. For example, the timing control in connection with the RFpower level and the pulse duration can selectively increase one ormultiple chemical reaction paths compared to other reaction paths. Thereaction selectivity allows the RF pulsed plasma reactor to bettercontrol reaction stoichiometry in material synthesis and film depositionand to provide a mechanism of crystal phase control in polymorphicmaterials synthesis. In addition, the pulsed flow of activated reactivematerials onto the target substrate can fundamentally alter theinteraction between the reactive radicals from the plasmas and thesubstrate surface layer. In the case of surface deposition, the reactivematerials flow onto the target substrate and adsorb on the surface area.Since the reactive materials flow stops after the pulse operation, theaffected surface layer can undergo relaxation prior to the next wave ofreactive materials flow. By controlling the time lapse between therepetitively pulsed reactive materials flow, it is possible to controlthe property of the surface deposition. Similar mechanism can work forsurface layer removal and surface treatment process.

FIG. 2 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor. In this reactor, the inlet area may have twoinlets 201, 210 wherein one inlet is for a carrier gas and the otherinlet is for the precursor materials. The rest of the reactor is thesame and has the same reference numbers (which are therefore notdescribed in detail here) as the reactor in FIG. 1 that generates aplasma 103. FIG. 3A illustrates an embodiment of the radio frequencypulsed inductive plasma reactor with a substrate 302 that is inside thereactor chamber for deposition, coating, surface modification andtreatment, surface removal and/or nano-device fabrication on thesubstrate. The rest of the reactor is the same and has the samereference numbers (which are therefore not described in detail here) asthe reactor in FIG. 1 that generates a plasma 103. FIG. 3B illustratesan embodiment of the radio frequency pulsed inductive plasma reactorwith a moveable substrate 320 and a lock load system 318 for deposition,coating, surface modification and treatment, surface removal and/ornano-device fabrication on the substrate. The rest of the reactor is thesame and has the same reference numbers (which are therefore notdescribed in detail here) as the reactor in FIG. 1 that generates aplasma 103. The load lock system transports the substrate in and out ofthe reactor chamber and may have a set of vacuum locks 319. Thisembodiment may be used to move large sized substrates into/out of thereactor chamber.

FIG. 4 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor having a material collection system forcollecting nano materials 402 produced inside of the reactor from theprecursors. The material collection system collects different materialsgenerated during the plasma generator and sustainment process. Thematerial collection system may be one or more filters 404 and/or one ormore collectors 406. The material collection system may capturenanoparticles, nanofibers, nanowires, nanocrystals such as quantum dotsand nanophosphors synthesized from the reactor. The rest of the reactoris the same and has the same reference numbers (which are therefore notdescribed in detail here) as the reactor in FIG. 1 that generates aplasma 103.

FIG. 5 illustrates another embodiment of the radio frequency pulsedinductive plasma reactor with dynamic RF frequency tuning 500 to improveRF coupling after breakdown and plasma generation. The rest of thereactor is the same and has the same reference numbers (which aretherefore not described in detail here) as the reactor in FIG. 1 thatgenerates a plasma 103. After the breakdown, the loading of the antennaby the plasma will change the value of the antenna inductance, whichwill cause the resonance frequency to shift to a different valuecompared to the pre-breakdown. In steady-state rf plasmas, a tuningnetwork typically adjusts the capacitor value in order to compensate thechange in resonance frequency, which takes more than 10 ms response timedue to its mechanical nature of the adjustment. As a result, thetraditional approach to adjust the resonance frequency is not suitablefor the pulsed rf plasma generation. Instead, this invention utilizesthe change in rf frequency of the power system, triggered by the onsetof plasma formation. One of the benefits of using solid state switchingrf system is its ability to change the rf frequency by simply changingthe switching pulse timing. In the case of upward resonance frequencyshift, the control system timing can be pre-programmed to increase theswitching frequency after the breakdown to compensate the change in theantenna inductance by the plasma after breakdown.

FIGS. 6A-6C illustrate other embodiments of the radio frequency pulsedinductive plasma reactor that has dual pulsed RF power systems 108, 602.The rest of the reactor is the same and has the same reference numbers(which are therefore not described in detail here) as the reactor inFIG. 1 that generates a plasma 103. The first RF power system mayinclude elements 107-109 and the second may include elements 602, 604and 606 (pulsed signal, RF source and antenna). FIG. 6C illustratesanother embodiment in which the two antennas 107, 604 may be both drivenby a common RF source 108 that generates the pulsed RF signal 109. Therest of the reactor in FIG. 6C is the same and has the same referencenumbers (which are therefore not described in detail here) as thereactor in FIG. 1 that generates a plasma 103. In some implementations,the antennas for each pulsed RF power source are interposed as shown inFIG. 6A, but may also be configured differently. In this embodiment, afirst pulsed RF power source may be used to initiate the plasmageneration and then the second pulsed RF power source may be used tosustain the plasma once it is generated.

FIGS. 7A-7E illustrate process for generating pulse plasma using thepulsed plasma reactor. A pulsed plasma spray source is another exampleof pulse plasma source and timed operation of gas injection and pumping.In conventional plasma spray, the plasma source needs to operate atatmospheric pressure or above atmospheric pressure in order to transportplasma activated reactive species onto the target surface located in theambient environment. For example, a thermal plasma spray is one exampleof such device that thermally activates precursor materials in theplasma reaction volume and the activated materials such as melted metalsand ceramics are sprayed onto a target surface located outside thereactor in an ambient pressure. In a pulsed plasma spray source, RFpulsed plasma operation can occur at gas pressures below atmosphericpressure, while thermally and chemically activated materials can besprayed onto a target surface located in an ambient pressure.

Step 1 (FIG. 7A): At t₀, the plasma reactor volume is sealed by a seal707 that is closed from the ambient environment and the gas pumping isprovided to maintain low gas pressure in the reactor volume. No RF poweris being applied in this step. During this process, the inlet valve 201,210 may be closed (filled pattern) and the pumping valve 105 may be open(unfilled pattern.)

Step 2 (FIG. 7B): Between t₁ and t₂, carrier gas and precursor materialsare introduced into the plasma reactor with operation of pulsed inletvalves 201, 210. The gas pressure in the reactor is maintained between 5torr and 500 torr by the balance between the gas inflow and the pumping,while being sealed by the seal 707 from the ambient pressure. However,the pumping valve 105 may be open.

Step 3 (FIG. 7C): Between t₂ and t₃, pulsed RF powers are applied andgenerate plasmas 103. The plasma can activate precursor materials andgenerate reactive species. It is noted that rapid plasma heating willincrease the pressure in the reactor volume from its initial pressureand the seal 707 is still closed and thus no materials are coming out ofan exit nozzle 708. In this process, the inlet valve 201, 210 may beopen and the pumping valve 105 may be open.

Step 4 (FIG. 7D): Between t₃ and t₄, the activated materials by thepulsed plasmas leave the reactor and flow toward the outlet. With theuse of rotating pneumatic seals 707, an exit nozzle 708 is open, whichallow the activated materials 713 to flow onto a target surface 706located outside the reactor at ambient pressure. At the same time, thegas pumping path is closed. If the reactor gas pressure during thepulsed plasma operation exceeds the atmospheric pressure, the outflow ofthe reactive materials can be directed toward the target surface withoutadditional gas injection. If the reactor gas pressure during the pulsedplasma operation is below the atmospheric pressure, an additional gasvalve in the reactor upstream is open in order to increase the reactorgas pressure and to generate outward flow to the ambient environmentthrough the exit nozzle.

Step 5 (FIG. 7E): Between t₄ and t₅, once the majority of reactivematerials are sprayed onto a target surface, the seal 707 is thenclosed, while the gas pumping is resumed. Once the gas pressure in thereactor drops below desired set point after sufficient gas pumping, thecycle repeats from step 1 to step 5 with the repetition rate between 1Hz and 100 Hz.

The above described pulsed plasma spray allows the various in-situplasma treatments such as deposition, surface modification, surfacelayer removal and patterning, for large objects and the target materialsthat cannot be contained in the plasma reactor. Furthermore, since thepulsed plasma generates little thermal heat, the pulsed plasma spray canbe applied to any heat sensitive materials such as glass, plastics,polymers, fabric, paper, fiberglass, and composite materials. Forexample, the pulsed plasma spray can apply protective coatings on theheat sensitive large scale glass reinforced plastic structure materialssuch as wind turbine blades and marine vessel body.

Experimental and Theoretical Results

Experimental data for RF pulsed plasma generation between 0.8-1.5 MHz, 1torr to 10 torr argon and helium was performed. The pulse durationduring the experiments of 10-100 μs range has been demonstrated.Repetitive power system is also demonstrated.

FIG. 8 shows Paschen curve data for the radio frequency pulsed inductiveplasma reactor for common carrier gases such as helium (He), neon (Ne),argon (Ar), hydrogen (H₂) and nitrogen (N₂), indicating that highvoltage at the antenna (shown along the vertical axis) is needed toachieve rapid gas breakdown at high gas pressures.

FIG. 9 show computer simulation results with breakdown delay time as afunction of azimuthal electrical strength of the antenna for gaspressure in argon using 1 MHz RF power. More particularly, FIG. 9 showsbreakdown delay time as a function of azimuthal electrical fieldstrength of the antenna for gas pressure in argon using 1 MHz RF power.The results are from a one-dimensional particle-in-cell (PIC) simulationwith Monte Carlo collision (MCC) treatment for electron gas collisionsincluding ionization, excitation, and elastic momentum and energytransfer collision. A XPDC1 code, described in J. P. Verboncoeur, M. V.Alves, V. Vahedi, and C. K. Birdsall, J. Comput. Phys. 104(2), 321(1993), was used for the PIC-MCC simulation for a one-dimensional radialslice of a cylindrical RF pulsed discharge reactor with a single turncoil antenna for a 1.5 cm reactor radius and 9.4 cm azimuthalcircumference. In order to include the effect of the azimuthal electricfield for inductively coupled discharges, Maxwell's equations are solvedfor the azimuthal electric field with the plasma conductivity andcoupled to the PIC-MCC simulation as reported by Lee and Verboncoeur(described in H. J. Lee and J. P. Verboncoeur, Phys. Plasmas 9(11), 4804(2002). In this simulation, only the initial breakdown phase wasconsidered, and thus the evolution of excited states and the radiationtransport were neglected. The simulation starts with an initiallyuniform plasma density of 1×10⁶ cm⁻³ in the reactor volume and thebreakdown is defined when the plasma density is increased by 1,000 foldsto 1×10⁹ cm⁻³ from the avalanche ionization with the energy transferfrom the RF electric fields to the plasma. In order to save thecomputation time, the maximum breakdown delay time is chosen at 10 μs.As described above, rapid breakdown is important for pulsed RF plasmasource operation. The data points indicating 10 μs breakdown delay timeare the case when no breakdown is achieved within the first 10 μs afterthe RF power is applied to the system. The results from the computersimulation agree in general with the Paschen curve for argon shown inFIG. 8.

FIGS. 10A-10D shows the time history of rf pulsed plasma generation atfour different gas pressures of argon, 1.2 torr, 3.2 torr, 6.5 torr and9.8 torr. The rf voltage on the antenna is shown by trace 1, which ismeasured using high voltage probe with 1,000× voltage division. The rfcurrent across the antenna is shown using trace 3, which is measuredusing Pearson current monitor with 1 kA/1V sensitivity. The plasma lightemission is shown using trace 2, which is measured by a fast photodiodewith 50 ohm termination. The reactor size is 48 mm diameter and an8-turn antenna was constructed with the high voltage insulated 12 gaugewire, resulting in a vacuum inductance of 4 μH. Rf switching powersystem operates with the switch voltages of +130 V and −130V at the IGBTswitches in a half bridge configuration. With the use of 10 nF highvoltage tuning capacitor, the rf switching frequency was chosen at 800kHz and no dynamic tuning was performed after breakdown. Once the rfswitching system is turned on, it takes approximately 50 μs to reach apeak rf voltage of 16 kV, a peak to peak value, at the antenna. It isnote that this turn on time for the series circuit can be controlled andreduced by adjusting the maximum IGBT surge current. After the seriesresonance circuit reaches its peak voltage, the breakdown occurs in thereactor from the electron avalanche. At 1.2 torr, the delay time is only7 μs, while it increases to 20 μs at 3.2 torr, 53 μs at 6.5 torr, 100 μsat 9.8 torr, showing the technical challenge related to the rapid highpressure gas breakdown using rf pulse power. After the breakdown, thedischarge can be sustained by maintaining rf pulse power to the antenna.Thus, the pulse plasma duration can be easily programmed by controllingthe total pulse duration using the following relation, t_(plasma)=t^(rf)_(total)−t^(rf) _(charging)−t_(breakdown), where t_(plasma) is the pulseplasma duration, t^(rf) _(total) is the total duration of rf switchingcircuit, t^(rf) _(charging) is the initial series resonance chargingtime, and t_(breakdown) is the time delay of gas breakdown. It is notedthat the plasma loading changes the resonance circuit property afterbreakdown and the de-tuning of resonance is observed from thesignificant reduction in antenna voltage and current. As discussedpreviously, it is possible to re-tune the resonance circuit by adjustingthe rf frequency of the switching power supply though we did notattempted dynamic tuning of the rf switching supply in this figure.

FIGS. 11A-11C show examples of a resonance circuit that may be used inthe pulsed plasma reactor. Specifically, FIG. 11A shows an example of aserial resonance circuit 1100 that may be used to generate the pulsed RFpower to the antenna. The series resonance circuit 1100 may include RFpower 1102, a series capacitor Cs 1104, an RF antenna Ls 1106 and aresistor Rs 1108 (for plasma loading and parasitic resistance) that areinterconnected to each other in series as shown in FIG. 11A.

FIG. 11B shows an example of a parallel resonance circuit 1110. Theparallel resonance circuit 1110 may include RF power 1112, a parallelcapacitor Cp 1114, an RF antenna Ls 1116 and a resistor Rp 1118 (forplasma loading and parasitic resistance) in which the capacitor Cp,antenna and resistor are connected in parallel to the RF power.

FIG. 11C shows an example of a hybrid resonance circuit 1120 that hasone RF power source and two antennas (R_(s, plasma) and R_(p, plasma)).The hybrid resonance circuit 1120 may include RF power 1122, a parallelcapacitor Cp 1124 in parallel with the RF power, parallel RF antenna Lp1126 for plasma sustainment and resistor Rp 1128 in series with eachother and in parallel with the RF power and the capacitor. The hybridresonance circuit 1120 may also include a series capacitor Cs 1130 andLs and R_(plasma) in series with each other and both in parallel withthe RF power. In the hybrid resonance circuit, an initial breakdownoccurs via the series resonance network using the first antenna and theplasma sustainment occurs via the parallel resonance network using thesecond antenna.

While the foregoing has been with reference to a particular embodimentof the invention, it will be appreciated by those skilled in the artthat changes in this embodiment may be made without departing from theprinciples and spirit of the disclosure, the scope of which is definedby the appended claims.

1. An apparatus of generating plasma, comprising: a pulsed radiofrequency source that is driven with a pulse duration and generates apulsed radio frequency signal; a reactor chamber that contains a carriergas and one or more reactive precursors with a pressure of between 1torr and 2,000 torr; an inductive antenna, coupled to the pulsed radiofrequency source, that surrounds a portion of the reactor chamber; andwherein the pulsed radio frequency signal is delivered to the inductiveantenna that initiates a breakdown of the carrier gas and one or morereactive precursors and generates a plasma due to the breakdown of thecarrier gas and one or more reactive precursors.
 2. The apparatus ofclaim 1, wherein the pulse duration is greater than 10 μs.
 3. Theapparatus of claim 1, wherein the pulse duration is less than 10 ms. 4.The apparatus of claim 1, wherein the pulse duration is between 10 μsand 10 ms.
 5. The apparatus of claim 1, wherein the radio frequency isgreater than 50 kHz.
 6. The apparatus of claim 1, wherein the radiofrequency is less than 10 MHz.
 7. The apparatus of claim 1, wherein theinductive antenna is a cylindrical coil of a single turn or multi-turns.8. The apparatus of claim 1, wherein the pulsed radio frequency sourcechanges its frequency after initiation of the plasma.
 9. The apparatusof claim 1 further comprising a second pulsed radio frequency sourcethat is driven with a pulse duration and generates a second pulsed radiofrequency signal and a second inductive antenna, coupled to the secondpulsed radio frequency source, that surrounds the portion of the reactorchamber, wherein the second pulsed radio frequency signal sustains theplasma.
 10. The apparatus of claim 1 further comprising an inlet systemto provide the carrier gases and the one or more reactive precursorsinto the reactor chamber in a repetitively pulsed manner with the dutyfactor between 1% and 100%.
 11. The apparatus of claim 1 furthercomprising a pumping system to maintain the pressure in the reactorchamber in a repetitively pulsed manner with the duty factor between 1%and 100%.
 12. The apparatus of claim 1, wherein the pulsed radiofrequency source has a repetition rate from 1 Hz to 1 kHz.
 13. Theapparatus of claim 12, wherein the repetitively pulsed radio frequencysource operates at a duty factor from 0.1% and 20%.
 14. The apparatus ofclaim 1 further comprising a material collection system to collectmaterials formed in the reactor chamber.
 15. The apparatus of claim 1further comprising a substrate component with target materials to betreated by the one or more reactive precursors that are activated by theplasma.
 16. The apparatus of claim 15, wherein the substrate componentis one of at an output of the reactor chamber and within the reactorchamber.
 17. The apparatus of claim 15, wherein the substrate componentis located on the moveable stage where it is continuously transportedfrom outside the reactor chamber into the reactor chamber to be treatedby the one or more reactive precursors that are activated by the plasmaand is moved out of the reactor chamber upon completion of thetreatment.
 18. The apparatus of claim 1, wherein the rf power to theplasma is more than 10 kW during the pulse.
 19. The apparatus of claim1, wherein the rf power to the plasma is less than 10 MW during thepulse.
 20. The apparatus of claim 18, wherein one or more reactiveprecursor materials are in one of a gaseous phase, a liquid phase or asolid phase.
 21. The apparatus of claim 20, wherein the one or morereactive precursor materials are complex compound materials includingcopper indium gallium selenide and YAG.
 22. The apparatus of claim 20,wherein the linear size of the solid precursor materials is less than0.1 mm.
 23. The apparatus of claim 20, wherein the linear size of thesolid precursor materials is more than 10 nm.
 24. The apparatus of claim20, wherein the linear size of the liquid precursor materials is lessthan 0.1 mm.
 25. The apparatus of claim 20, wherein the linear size ofthe liquid precursor materials is more than 10 nm.
 26. The apparatus ofclaim 20, wherein one or more reactive precursor materials are reactivegases containing hydrogen, oxygen, nitrogen, fluorine, chlorine, sulfur,phosphor and hydrocarbon.
 27. The apparatus of claim 20, wherein one ormore reactive precursor materials are acids, bases, polymers, metals,ceramics, and composite materials.
 28. The apparatus of generatingplasma, comprising: a pulsed radio frequency source that is driven witha pulse duration and generates a pulsed radio frequency signal; areactor chamber that contains a carrier gas and one or more reactiveprecursors with a pressure of between 1 torr and 2000 torr; an inductiveantenna, coupled to the pulsed radio frequency source, that surrounds aportion of the reactor chamber; a material exhaust system, connected tothe reactor chamber, that regulate the materials flow out of thereactor; and wherein the pulsed radio frequency signal is delivered tothe inductive antenna that initiates a breakdown of the carrier gas andone or more reactive precursors and generates a plasma due to thebreakdown of the carrier gas and one or more reactive precursors andactivate one or more reactive precursors by the plasma and exhaust thematerial flow from one or more reactive precursors that are activated bythe plasma out of the reactor.
 29. The apparatus of claim 28 furthercomprising a pulsed material exhaust system to allow the flow of one ormore reactive precursors that are activated by the plasma out of thereactor in a repetitively pulsed manner with the duty factor between 1%and 99% and with the repetition frequency between 1 Hz and 100 Hz. 30.The apparatus of claim 28 further comprising a pumping system tomaintain the pressure in the reactor chamber in a repetitively pulsedmanner with the duty factor between 1% and 99%.
 31. The apparatus ofclaim 28 further comprising an inlet system to provide the carrier gasesinto the reactor chamber in a repetitively pulsed manner with the dutyfactor between 1% and 99%.
 32. The apparatus of claim 28 furthercomprising an inlet system to provide one or more reactive precursorsinto the reactor chamber in a repetitively pulsed manner with the dutyfactor between 1% and 99%;
 33. The apparatus of claim 28 furthercomprising a nozzle to direct the flow of one or more reactiveprecursors that are activated by the plasma onto the target surfaceslocated outside the reactor in an ambient pressure to be treated by theone or more reactive precursors that are activated by the plasma. 34.The apparatus of claim 28 further comprising thermally sensitive targetmaterials and materials for plasma assisted material deposition andcoating, surface removal, surface activation and surface propertymodification.
 35. A method for generating plasma in a reactor chamberthat contains a carrier gas and one or more reactive precursors with apressure of between 1 torr and 2,000 torr, the method comprising:generating, by a pulsed radio frequency source that is driven with apulse duration, a pulsed radio frequency signal; delivering the pulsedradio frequency signal to an inductive antenna coupled to the pulsedradio frequency source and surrounding a portion of the reactor chamber;and initiating, by the pulsed radio frequency signal delivered to theinductive antenna, in the reactor chamber a breakdown of the carrier gasand one or more reactive precursors to generate a plasma.
 36. The methodof claim 35, wherein generating the pulsed radio frequency signalfurther comprises generating the pulsed radio frequency signal with apulse duration that is greater than 10 μs.
 37. The method of claim 35,wherein generating the pulsed radio frequency signal further comprisesgenerating the pulsed radio frequency signal with a pulse duration thatis less than 10 ms.
 38. The method of claim 35, wherein generating thepulsed radio frequency signal further comprises generating the pulsedradio frequency signal with a pulse duration that is between 10 μs and10 ms.
 39. The method of claim 35 further comprising changing, by thepulsed radio frequency source, its frequency after initiation of theplasma.
 40. The method of claim 35 further comprising generating, by asecond pulsed radio frequency source, a second pulsed radio frequencysignal and sustaining, by an inductive antenna coupled to the secondpulsed radio frequency source, the plasma by the second pulsed radiofrequency signal.
 41. The method of claim 35 further comprisingproviding, by an inlet system, the carrier gases and the one or morereactive precursors into the reactor chamber in a repetitively pulsedmanner with the duty factor between 1% and 100%.
 42. The method of claim35 further comprising maintaining, by a pumping system, a pressure inthe reactor chamber in a repetitively pulsed manner with the duty factorbetween 1% and 100%.
 43. The method of claim 35 further comprisingcollecting, by a material collection system, materials formed in thereactor chamber.
 44. The method of claim 34 further comprising treatingtarget materials using the one or more reactive precursors that areactivated by the plasma.
 45. The method of claim 44, wherein one or morereactive precursor materials are in one of a gaseous phase, a liquidphase or a solid phase.
 46. The method of claim 44, wherein one or morereactive precursor materials is one of a reactive gas containinghydrogen, oxygen, nitrogen, fluorine, chlorine, sulfur, phosphor andhydrocarbon.
 47. The method of claim 44, wherein one or more reactiveprecursor materials are acids, bases, polymers, metals, ceramics, andcomposite materials.
 48. A method for generating plasma in a reactorchamber that contains a carrier gas and one or more reactive precursorswith a pressure of between 1 torr and 2,000 torr, the method comprising:generating, by a pulsed radio frequency source that is driven with apulse duration, a pulsed radio frequency signal; delivering the pulsedradio frequency signal to an inductive antenna coupled to the pulsedradio frequency source and surrounding a portion of the reactor chamber;initiating, by the pulsed radio frequency signal delivered to theinductive antenna, in the reactor chamber a breakdown of the carrier gasand one or more reactive precursors to generate a plasma that activatesthe one or more reactive precursors; and exhausting, by a materialexhaust system connected to the reactor chamber, the material flow fromone or more activated reactive precursors out of the reactor chamber.49. The method of claim 48, wherein exhausting the material flow furthercomprises exhausting the material flow from one or more activatedreactive precursors out of the reactor chamber in a repetitively pulsedmanner with a duty factor between 1% and 99% and with a repetitionfrequency between 1 Hz and 100 Hz.
 50. The method of claim 48 furthercomprising maintaining, by a pumping system, a pressure in the reactorchamber in a repetitively pulsed manner with the duty factor between 1%and 100%.
 51. The method of claim 48 further comprising providing, by aninlet system, the carrier gases and the one or more reactive precursorsinto the reactor chamber in a repetitively pulsed manner with the dutyfactor between 1% and 100%.
 52. The method of claim 48 furthercomprising directing, using a nozzle, a flow of one or more reactiveprecursors onto a target surface located outside the reactor chamber inan ambient pressure so that the target surface and the one or morereactive precursors are activated by the plasma when the target surfaceis in the reactor chamber.